Flow control servo valve with dynamic load pressure feedback



H. B. KOLM, JR

July 2, 1963 FLOW CONTROL SERVO VALVE WITH DYNAMIC LOAD PRESSURE FEEDBACK @7o/ways.

United States Patent O M'ce 3,095,906 FLOW CNTROL SERV@ VALVE WETH DYNAMIC LOAD PRESSURE FEEDBACK Harvard B. Kolm, ir., East Aurora, NSY., assigner to Moog Servocontrols, Inc., a corporation of New York Filed Mar. 5, 1959, Ser. No. 797,487 2 Claims. (Cl. IS7-625.62)

This invention relates to improvements in electrically operated, uid handling, servo Valves, and more particularly to a flow control servo valve with high frequency pass load pressure feedback. Such an improved valve may be called a dynamic pressure feedback servo valve.

In U,S. Patent No. 2,767,689 there is disclosed an electrohydraulic flow control servo valve which produces a valve spool displacement proportionate to an electrical signal input, irrespective of load reaction.

Also known is an electrohydraulic pressure-flow control servo valve which controls spool displacement jointly by the electrical signal input and by the reaction of the load with which the valve is associated. Such a pressureflow valve contributes effective damping in dynamically loaded servo systems. However, the valve responds to static as well as dynamic load pressures so that the valve spool tends to move for any and all load reactions. Such a valve is disclosed in the pending application of Lewis H. Geyer, Serial No. 797,488, `filed March 5, 1959, and entitled Pressure-Flow Servo Valve, now Patent No. 2,964,059.

The primary object of the present invention is to provide a dynamic pressure feedback servo valve which contributes damping in dynamically loaded servo systems while retaining the desirable system static stiffness characteristics of non-load-pressure sensitive servo valves. Both of these desirable characteristics are made possible by Iusing load pressure feedback through a frequency sensitive network. Essentially, the improved valve has the high pressure gain characteristics of a flow control valve in the steady state and the load sensitive characteristics of a pressure-flow valve at higher frequencies. The frequency sensitive network approximates a high pass, firstorder filter, and has a corner frequency which is typically set somewhere between an octave and a decade below the resonant frequency of the load. The frequency determining parameters of this filter may be varied to suit particular system requirements. The sensitivity of flow to load pressure which occurs due to pressure `feedback may be tailored to provide the equivalent damping required for a particular load.

Other objects and advan-tages will be apparent from the following detailed description and accompanying drawing in which:

FIG. l is a vertical central sectional view through a dynamic pressure feedback electrohydraulic servo valve of preferred construction and embodying the present invention and illustrating Vthe internal construction of the valve in a more or less diagrammatic manner and with the valve spool in a neutral position.

FIG. 2 is a curve depicting the relationship between ow through and pressure applied to a restricted orifice.

FIG. 3 is a curve depicting the corner lfrequency of a -typical frequency sensitive network for a valve constructed in accordance with the principles of the present invention, when varying the amplitude of the load pressure differential oscillated at a given frequency.

FIG. 4 is a curve depicting the measured relationship between amplitude ratio and frequency of oscillation of load pressure differential and thereby showing the gain of such typical frequency sensitive network, this figure also including a curve depicting the phase relationship between feedback pressure differential and load pressure differential for such a valve at varying frequencies.

3,095,906 Patented July 2, 1963 Referring to the drawing, the valve is shown as having a body 10 internally formed with a horizontally elongated central chamber '11 connected at opposite ends severally through reduced bores 12, 12a with end chambers 13, 13a, respectively. All of these chambers and the intermediate'red-uced bores are cylindrical and coaxial. The body wall of the central chamber 11 is shown as provided with three annular grooves 14, 15 and 16 at axially spaced intervals therealong. The right groove 14 is shown as communicating with a return or drain port 18 and the left groove 16 via a iiuid channel 17 is also shown as communicating with the drain port 18. The center groove 15, somewhat longer axially than the other two, is shown as communicating with a pressure port 19. 'Ihe portion of the central chamber 11 between the grooves 14 and 15 is shown as being in communication with an actuating port 20. The portion of the central chamber 11 between the grooves 15 and 16 is shown as being in communication with a second actuating port 21.

The -return port 18 and pressure port 19 are connected with -any suitable hydraulic system (not shown). The actuating ports 2f) and 211 are connected with external hydraulic machinery indicated schematically as a piston and cylinder actuator and represented by the numeral 22.

Thus, the servo valve has inlet, outlet and actuating ports for supplying a receiving fluid to and from the actuator 22 to be actuated.

A valve spool 23 is slidably arranged in the central chamber 11 and is shown as having a central lobe 24 and two end lobes 25 and 26. The central lobe 24 has an axial length corresponding to that of the central groove 15. The end lobes 25 and 26 are longer axially than the width of the end grooves 14 and 16. When the valve spool 23 is in its centered or neutral position shown in FIG. 1, the central lob-e 24 covers the center groove 15, and the end lobes 25 and 26 cover the end grooves 14 and 16, respectively, with the inner ends of these end lobes opposite the inner end of these end grooves. In this centered or neutral position of the valve spool 23, neither actuating port 20 or 21 has communication with either the pressure port 19 or the drain port 18.

Similar piston heads 28 and 28a are shown severally as being slidably arranged in the intermediate reduced bores 12 and 12a, respectively. These piston heads are arranged at opposite ends of the valve spool and are axially spaced outwardly therefrom. Each piston head is shown as being integrally connected to the correspondmg end of the valve spool 23 by a stern, such stern being indicated at 29 for the piston head 28 and at 29a for the other piston head 28a. The portion of the chamber 11 between the inner end face of the left piston head 28 and the outer end face 30 of the left end lobe 26 provides a hydraulic drive chamber 31. A similar drive chamber 31a, is provided between the inner end face of the right piston head 28a and the outer end face 30a of the right end lobe 25. The inner end of the left end chamber 13 is closed by the outer end face 32 of the left piston head 28, and similarly the inner end of the right end chamber 13a is closed by the outer end face 32a of the right piston head 28a.

A helical compression spring 33 is shown arranged in the left end chamber 13 and suitably operatively interposed between the fixed outer end wall of this chamber and the outer end face 32 of the movable left piston head 2S. A similar spring 33a is shown as arranged in the right end chamber 13a and between the fixed outer wall thereof and the outer end face 32a of the movable right piston head 28a.

Means are provided for producing a pressure differential in the spool drive chambers 31 and 31a so as to drive the spool 23 hydraulically. Such means are shown as including an electrohydraulic amplifier iirst stage, represented generally by the numeral 34, which produces any output pressure differential proportionate to an electrical signal input.

The preferred electrohydraulic amplifier 34 is shown as having a solenoid or torque motor 3S arranged in a compartment 36 in the valve body, and adapted to move a pressure regulator member 38. A pair of nozzles 39 and 40 are arranged in a separate compartment 41 which serves as a sump chamber. The end portion of the pressure regulator member 38 remote from the torque motor 35 extends between the discharge openings of the nozzles 39 and 40 and in a variably spaced relation thereto. The pressure regulator member 38 intermediate its ends is mounted on a tiexure tube 37. The position of the lower or liapper portion of the pressure regulator member with respect to the discharge openings of the nozzles 39 and v 40 provides variable annular orifices which develop a pressure differential within the nozzle chambers or interiors.

It will be observed that the solenoid or torque motor 35 is isolated from the sump chamber 41 into which the nozzles 39 and 40 discharge fluid. Such a dry solenoid type of electrohydraulic amplifier is more fully described as to construction and operation in the patent application of William C. Moog, Jr., Serial No. 560,573, filed Ianuary 23, 1956, and entitled Fluid Control Valve in which a mechanical motion is transmitted from a dry region to a pressurized fluid tilled region. Instead of the preferred dry solenoid type of first stage amplifier shown, an immersed solenoid type may be employedV such as is fully described in the aforementioned -Patent No. 2,767,689. Both such types of electrohydraulic amplifiersl are of the balanced nozzle design and regardless of which type is employed in the practice of the present invention, provide an output pressure dilferential proportionate to the electrical signal input to the solenoid or torque motor 35. Such hydraulic ampliiiers provide a high force level output push-pull hydraulic drive with completely frictionless operation and excellent Vdynamic performance while requiring very little signal input current.

The construction of each of the electrohydraulic arnpliers referred to above may be herein described as comprising a polarized electrical force motor including spaced pole pieces, permanent magnet means and electromagnetic means associated with the pole pieces, and having for its armature and extending between the pole pieces a apper means arranged to move with mechanical spring restraint, the centering force gradient of which is substantially cancelled by the decentering force gradient of the permanent magnet means, and a pair of xed nozzles in spaced relation to different surface portions of the apper to provide variable oriiices for discharging uid, variations in the position of the flapper relative to the nozzles resulting in a differential pressure between the nozzle chambers, the flapper moving to reduce to zero the summation of the moments acting upon it.

Means are provided for supplying the electrohydraulic amplifier 34 with fluid derived from the hydraulic system connected to the pressure and return ports 19 and 18, respectively. As shown, the uid feed is by means of independent branch fluid feed channels 42 and 43 severally communicating at one end with the chambers or interiors of the nozzles 39 and 40, respectively. At their other endsV the channels 42 and 43 communicate with a main fluid feed channel 44 which leads to the annular pressure groove 15, in turn connected to the pressurel port 19. The channels 43 and 42 are shown severally as having restrictions 45 and 45a, respectively, therein so that fluid supplied to the nozzles is at a lower pressure than the hydraulic system supply pressure applied to the port 19. A fluid drainv channel 46 having a restriction 48 therein is shown as placing the sump charnber 41 in communication with the annular drain groove 14, in turn connected to the return port 18.

Means are shown for applying the pressure differential created by the nozzles 39 and 40 to the spool drive chambers 31 and 31a so that this pressure differential can be utilized to drive hydraulically the valve spool 23. Such means are shown as including fluid channels 49 and S0. The channel 49 at oneV end communicates with the fluid feed branch channel 42 on the downstream side of the restriction 45a therein, and the opposite end of the channel 49 communicates with the right spool drive chamber 31a. The iiuid channel 5t) is shown as communicating with the uid feed branch channel 43 on the downstream side of the restriction 45 therein, and the other end of this channel 5b communicates with the left drive chamber 31. Thus, the respective output pressures developed by the nozzles 39 and 40 are applied severally to the end faces 30a and 30, respectively, at opposite ends of the valve spool 23.

In accordance with the principles of the present invention, the pressure differential across the load, as sensed in the actuating ports or load lines 20 and 21 connected to the actuator 22, is fed back to the valve spool 23 through a frequency sensitive network in such manner that the load pressure diiferential is in :opposition to the hydraulic drive on the valve spool due to the pressure differential in the drive chambers 31 and 31a. As shown, such frequency sensitive network comprises pressure drop producing means including orifice or restrictor 51 providing a hydraulic resistance, and also comprises resiliently rest-rained fluid volume transfer means includi-ng a spring retained piston or accumulator 52 providing a hydraulic capacitance. This piston 52 is shown as being slidably arranged in a vertically elonged cylindrical chamber 53 formed in the valve body 10. Two helical compression springs 54 and 55 are shown severally as being arranged on opposite sides of the piston 52. The upper spring 54 is suitably operatively interposed between the yupper closed end of the cylinder 53 and the upper end face of the piston 52. The lower spring 55 is suitably operatively interposed between the lower closed end of the cylinder 53 and the lower end face of the piston 52. Adjacent its upper end and above the piston 52, the cylinder 53 is interiorly connected by the channel 56 to the left end chamber 13. Adjacent its lower end and below the piston 52, the cylinder S3 is interiorly connected by the channel 58 to the actuating port or load line 20. A channel 59 having the orifice or restriction 51 therein, is shown as establishing communication between the channel 56 and the left actuating port or load line 21. A channel 66 is shown as establishing communication between the left actuating port or load line 21 and the right end chamber 13a.

Thus, the spring retained piston 52 is operatively arranged in a feedback line, consisting of the channels 56 and 5S, which connects the left end chamber v13 with the right actuating port 20. The channel 60 also serves as a feedback line which connects the other or left actuating port 21 with the other or aight end chamber 13a. The channel 59 having the restricted orifice 51 therein serves as a hydraulic resistance which is shunted in effect across the aforementioned feedback lines.

The spring retained piston 52 forming a hydraulic capacitance and the restricted orice 51 forming a hydraulic resistance in the network shown and described, provide a hydraulic high-pass lter which feeds back to the valve spool 23 load pressure differential directly at high frequencies but lesser proportions at lower frequencies. Thus, the network is operative to pass the load pressure variations more effectively above than below a predetermined frequency.

The network has an approximate transfer function relating output pressure differential (APo) to input pressure differential (APi) of the following form:

A PL TS Api-1+7'S 'The hydraulic resistance may be either of the orifice or viscous type. An orifice resistance is highly non-linear or non-proportionate as illustrated in FIG. 2, while a viscous resistance is. linear but extremely temperature sensitive. For reasons of simplicity an Orifice resistance 51 is preferred since its non-linear behavior is not important provided it performs its function.

A hydraulic resistance can be defined as a hydraulic damping element in which flow through the element is related to the pressure across the element. The effective resistance of such an element is the ratio of the pressure to the iiow, such that for an orifice, resistance can be expressed as:

R= hydraulic resistance (orifice damping effect) s Q=instantaneous flow through orifice @l-1) K=onfice flow coefficient A0=Oritice area (in.2)

P0=norrnal pressure drop across orice (constant) Y m' It is readily seen that the resistance value for an orifice is a non-linear function of the pressure amplitude. The relationship between pressure and ow (Q) is show-n by the solid line curve in FIG. 2 representing the equation Q=KA0\/AP and where the slope of a line from the origin to the curve at any point is expressed by Equation 2, from which it will be seen that as pressure amplitude increases, incremental flow changes decrease. The dashed line in FIG. 2 depicts the approximation that is made for the characteristics when the foregoing resistance Equation 2 is used for an orifice, assuming a specific value for pressure differential (AP).

A viscous or capillary resistance, on the other hand, -is linear but is dependent upon oil viscosity which varies with temperature. The value of hydraulic resistance for a capillary tube is obtained from the following equation:

where R=hydraulic resistance (#sec' ins" M=absolute viscosity (#5165 L=tube length (in.) D=internal diameter (in.)

The spring retained piston 52 forms a hydraulic capacitance which has the following value:

5 C=hydraulic capacitance where A=piston area (in) Ker-total spring rate The frequency at which the high frequency and low frequency amplitude asymptotes of the frequency response characteristic expressed by Equation 1 meet is known as the corner frequency (fc) and is defined by the reciprocal of the RC product, expressed by the following equation:

The resistance was provided by 4an -orifice having an area of 3.5 X5105 sq. in. and in applying Equation 2 consideration must be Vgiven to the fact that the pressure varied sinusoidally. -Due to the non-linear resistance of the orilice, the 45 phase point which, as known by those skilled in the art, indicates the corner frequency for the network, changes with pressure amplitude. The relationship between corner frequency for various input pressure amplitudes is shown by the curve in FIG. 3. The input pressure amplitude is the difference peak to peak between the pressures in the load lines 2li and 21 of the val-ve. These pressures were varied sinusoidally at various frequencies and ithe frequency at which the output pressure differential in end chamber 13 and 13a was 45 degrees out of phase with the input pressure differential in load lines 20 and 21, indicating .the corner frequency of the network, was detected and gives the plot in FIG. 3. Generally, it will be observed that the corner frequency decreases as the magnitude of the input pressure differential increases.

With a fixed input pressure differential olf 45()` p.s.i. peak to peak sinusoidally applied to the load lines 20 and Z1 and hence exciting the filter network the elements of which had the specic Values mentioned above, and varying the frequency of the excitation, the frequency response plotted in PIG. 4 resulted. In FIG. 4, the log of the amplitude ratio or the ratio of the output pressure differential AP,V to the Ainput pressure differential APi (constant at 450 psi. peak to peak) expressed in .decibels is plotted as the left handordinate against the log of the frequency expressed in #cycles per second of the input pressure differential as the abscissa. The phase angle between the input pressure differential and the `outputpressure different-iai is also plotted in FIG. 4 :against log frequency. The phase angle expressed in degreesj is the right hand 7 ordinate in FIG. 4 and indicates the phase of 'the output or feedback pressure differential with respect to the input or load pressure differential.

Inspection of FIG. 4 shows that the filter network w-ill not pass substantially full feedback pressures until the load or input pressure differential oscillates at a log frequency of about 10' c.p.s. At lower frequenciesthe load or input pressure dierential has been partially filtered out, the lower the frequency the more effective the filtering. Thus, at such low frequencies, the valve ywith its filter network provides static stiffness and operates in the nature of a flow control valve in ywhich valve spool displacement is proportionate only [to the signal input current.

Above the cut `off point of the lter network, that is, at log frequencies above about 10 cps. in the example given above, the lfull effect of the feedback pressure differential is felt by the valve spool and the valve operates in the nature of a pressure-flow valve in which valve spool displacement is controlled jointly by the signal input current and the load pressure differential. When the frequency of oscillation of the load pressure differential is above the eut ot point of the iiiter network, the spool end chamber 13 and load line 20 have the same pressure, and the other spool end chamber 13a and other load line 21 have the same pressure. Y

The piston head end faces '32 and 32a are of equa area, as lare the spool end faces 30 and 30a `although the faces 30, 30a are shown `as larger inarea than the faces 312, 32a.

Several forces act on the valve spool 23. VIgnoring flow reaction forces, one is the command pressure, being the pressure differential acting on vthe spool end faces 30 and 39a and proportionate to the electrical input signal to the electrohydraulic amplifier rst stage 3d. A second force consists of the spring forces applied by the spool end springs 33 :and 38a. A third Aforce is the lfeedback pressure differential applied to the end faces 32 and 32a.

- The -rst force is opposed lby the second and third forces.

It will be seen essentially that for every command input, the valve spool 23 moves to reduce the summation of the alb-ove lforces to zero. v

The effect is that -the functions of how and pressure control comb-ine to provide a valve characterized by having relatively -high initial flow commanded by a given electrical signal input which is reduced to no iiow as the pressure differential across the load is built up. By selecting the spool end areas Si), 30a, piston head areas 32, 32a, and the spring rate of the spool end springs 33, 33a, a valve can be designed yto have the damping characteristie desired.

Reverting to the hydraulic lter network, it will be apparent that more than one hydraulic capacitance producing device or l,accumulator means such as the spring retained piston 52 may be employed in the network to provide the total hydraulic capacitance desired. Likewise, the total hydraulic resistance desired can be provided -by more than one hydraulic resistance producing device or restrictor means such as the restricted orifice 51. Also, a number of other such combinations of resistance and capacitance elements could be devised by persons skilled in the art following the present teaching to perform :the same frequency selective function.

It is to be noted that when the pressure diterential across the load is not changing as sensed in the actuating ports 20 and 21, there is no fluid flow through the channels 56, 58, 59 and 60 and the pressure drop producing means 51 and with respect to the resiliently restrained uid volume transfer means 52--55 associated therewith so that the frequency sensitive network is free of Viluid iiow during such steady state conditions.

Suitable fluid contamina-tion filtering mean-s may, of course, be incorporated in :the hydraulic circuit of the present dynamic pressure feedback electrohydraulic servo valve at various places in order to maintain the hydraulic fluid free of foreign material. ,Such filtering means have not been illustrated herein in order Ito confine the disclosure to the Aessentials of the valve,

The present invention is applicable to electrically operated servo valves hand-ling not only hydraulic liquid but other liuids, such as compressed air or hot gases. By the term iiuid as used in the appended claims is meant either a liquid .or gas.

Moreover, while the load pressure differential passed by the frequency sensitive network has been illustrated in the accompanying drawings and described hereinabove as operatively fed back to the valve spool, it is to be clearly understood `that the present inventive concept :in its broadestvaspect contemplates any sui-table feedback arrangement by which the effective iiuid drive on the valve spool is reduced in response to the dynamic pressure differential in the actuating parts as a result of load reaction. For example, dynamic pressure feedback might be related to the apper of Vthe electrical force motor, rather than to the valve spool, Vin such a way as to work in opposition to the direction of displacement of the dapper induced by the electrical signal input. In both cases, the dapper moves to reduce to Zero the summation of the various moments acting upon it, and likewise the valve spool moves to reduce to zero the summation of the various forces acting upon it. Accordingly, the cmbodirnent of the invention shown and described is intended as illustrative only and not limitative of the inyention which as to scope is to be measured by the appended claims.

. What is claimed is:

1. In a servo valve, the combination comprising means providing actuating ports adapted to be connected to a fluid-operated load, a valve spool movable to control iiuid flow through said ports, means providing driving chambers at opposite ends of said spool, means providing feedback chambers at opposite ends of said spool, a iirst stage electrohydraulic amplifier arranged to produce in said driving chambers -a pressure differential output in response to an electrical input, and frequency sensitive load pressure feedback means operatively interposed between said ports and feedback chambers and including a first feedback channel connecting one of said feedback chambers to one of said ports, tiuid accumulator means operatively arranged in said iii-st feedback channel, a second feedback channel connecting the other of said feedback chambers directly to the other of said ports and flu-id restrictor means operatively establishing `iluid cornmunication between said first feedback channel and said other of said ports.

2. In a servo valve having actuating ports adapted to be connected to a fluid-operated load, ya rst stage electrohydraulic amplifier and a second stage Valve spool arranged -to be uidly driven by the output of said amplifier for control-ling the flow of fluid through said ports, the combinationtherewit-h of load pressure feedback means associated with said ports and including resiliently restrained fluid volume transfer means and pressure drop producing means arranged -to provide a frequency sensitive network for reducing the effective fluid drive on said spool in response to the dynamic pressure yditleaeritial in said ports as a result of load reaction and operative to pass the load pressure variations more effectively above than below a predetermined frequency, said network being free of uid iiow during steady state conditions.

References Cited in the file of this patent Y UNITED STATES PATENTS 2,550,723 Ross May 1, 1951 2,647,493 Whitehead et al. Aug. 4, 1953 2,688,314 Holm et al. Sept. 7, 1954 2,698,517 Witt Jan. 4, 1955 2,755,725 Parker July 3l, 1956 2,767,698 Moog Oct. 23, 1956 (Other references on following page) 9 UNITED STATES PATENTS Carson Apr. 30, 1957 Chubbuck July 2, 1957 Lloyd June 9, 1959 Keyt Oct. 20, 1959 5 10 Kohn Mar. 1, 1960 Moog Apr. 5, 1960 Westbury June 7, 1960 Lloyd Feb, 21, 1961 Blanton Nov. 20, 1962 

2. IN A SERVO VALVE HAVING ACTUATING PORTS ADAPTED TO BE CONNECTED TO A FLUID-OPERATED LOAD, A FIRST STAGE ELECTROHYDRAULIC AMPLIFIER AND A SECOND STAGE VALVE SPOOL ARRANGED TO BE FLUIDLY DRIVEN BY THE OUTPUT OF SAID AMPLIFIER FOR CONTROLLING THE FLOW OF FLUID THROUGH SAID PORTS, THE COMBINATION THEREWITH OF LOAD PRESSURE FEEDBACK MEANS ASSOCIATED WITH SAID PORTS AND INCLUDING RESLIENTLY RESTRAINED FLUID VOLUME TRANSFER MEANS AND PRESSURE DROP PRODUCING MEANS ARRANGED TO PROVIDE A FREQUENCY SENSITIVE NETWORK FOR REDUCING THE EFFECTIVE FLUID DRIVE ON SAID SPOOL IN RESPONSE TO THE DYNAMIC PRESSURE DIFFERENTIAL IN SAID PORTS AS A RESULT OF LOAD REACTION AND OPERATIVE TO PASS THE LOAD PRESSURE VARIATIONS MORE EFFECTIVELY ABOVE THAN BELOW A PREDETERMINED FREQUENCY, SAID NETWORK BEING FREE OF FLUID FLOW DURING STEADY STATE CONDITIONS. 